The Open Systems Interconnection Model (OSI Model or just OSI) is a layered abstract description for communications and computer network protocol design. It is a hierarchical structure generally comprised of seven layers that defines the requirements for communications between two computers. The model was defined by the International Standards Organization and was conceived to allow interoperability across the various platforms offered by vendors. Each layer of the OSI Model has the property that it only uses the functions of the layer below, and only exports functionality to the layer above. The layers generally include a physical layer (Layer 1), a data link layer (Layer 2), a network layer (Layer 3), a transport layer (Layer 4), a session layer (Layer 5), a presentation layer (Layer 6), and an application layer (Layer 7).
The physical layer, or Layer 1, defines all electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are physical-layer devices. Layer 1 networks, for example Next Generation Synchronous Optical Networks (NG-SONET), as are known in the art, are increasingly being used for transport of packet switched services. It is often desirable to use a lower speed (partial rate) transport link when connecting to a Packet Switched Network (PSN) to provide the required bandwidth at the least possible cost. An example of partial rate mapping is when Gigabit Ethernet interfaces are used on a NG SONET system (Ethernet over SONET) with an STS-3 provisioned across the SONET network, thus creating a speed mismatch condition (i.e., 1000 Mbps packet interface is mapped to 150 Mbps transport channel).
NG Add-Drop Multiplexers (ADMs), providing layer 1 transport are not typically Class of Service (CoS) aware—they also typically have limited queue sizes. While large queue sizes could help guarantee frame delivery performance, they introduce excessive delay and delay variation for premium (e.g., real-time) service classes. In addition, Layer 2 switch ports providing the connection from the PSN to the layer 1 transport network generally cannot perform the full set of traffic management functionality required (e.g., egress shaping, egress policing) to support the performance guarantees required for the enhanced service classes. As a result, when traffic from one or many PSN customers converges on one interface, it is highly probable that the layer 1 client interface will start dropping traffic indiscriminately, i.e., without regard to the priority or importance of the traffic being dropped. In such a condition, premium traffic (e.g., real-time) with strict Service Level Agreement (SLA) objectives will be dropped with the same probability as the low priority (best effort) traffic, a service class without any performance guarantees.
A common solution to this challenge is to substantially overprovision bandwidth across the Layer 1 transport system, (e.g. by mapping the packet interface bandwidth to a full-rate STS-n mapping). In the case of 1000 Mbps Gigabit Ethernet service, this equates to an STS-21 mapping at best. Needless to say, this is an expensive and inefficient solution for a customer desiring an aggregate committed information rate of only 150 Mbps.